“Graft copolymers” as used herein are macromolecules formed when polymer or copolymer chains are chemically attached as side chains to a polymeric backbone. Generally, the side chains are of a different polymeric composition than the backbone chain. Because graft copolymers often chemically combine unlike polymeric segments in one molecule, these copolymers have unique properties compared to the corresponding random analogues. These properties include, for example, mechanical film properties resulting from thermodynamically driven microphase separation of the polymer, and decreased melt viscosities resulting in part from the branched structure of the graft copolymer. With respect to the latter, reduced melt viscosities can advantageously improve processability of the polymer. See e.g., Hong-Quan Xie and Shi-Biao Zhou, J. Macromol. Sci.-Chem., A27(4), 491-507 (1990); Sebastian Roos, Axel H. E. Müller, Marita Kaufmann, Werner Siol and Clenens Auschra, “Applications of Anionic Polymerization Research”, R. P. Quirk, Ed., ACS Symp. Ser. 696, 208 (1998).
The term “comb copolymer,” as used herein, is a type of graft copolymer, where the polymeric backbone of the graft copolymer is linear, and each side chain of the graft copolymer is formed by a “macromonomer” that is grafted to the polymer backbone. “Macromonomers” are low molecular weight polymers having at least one functional group at the end of the polymer chain that can further polymerize with others monomers to yield comb copolymers. See e.g., Kawakami in the “Encyclopedia of Polymer Science and Engineering”, Vol. 9, pp. 195-204, John Wiley & Sons, New York, 1987. The term “linear,” as used herein, is meant to include polymers where minor amounts of branching has occurred through hydrogen abstraction that is normally observed in free radical polymerizations. The comb copolymers are commonly prepared by the free radical copolymerization of macromonomer with conventional monomer (e.g., ethylenically unsaturated monomers).
Comb copolymers prepared with water-insoluble macromonomers have been predominantly prepared using bulk and solution polymerization techniques. However, such processes undesirably use solvent or monomer as the medium in which the polymerization is conducted. Thus, efforts recently have focused on developing methods for preparing comb copolymers via an aqueous emulsion process.
One example, U.S. Pat. No. 5,247,040 to Amick et al., (“Amick”), discloses a two stage emulsion polymerization process for producing graft copolymers. In the first stage, a macromonomer is produced by polymerizing ethylenically unsaturated monomer in the presence of a mercapto-olefin compound. In the second stage, the resulting macromonomer is polymerized in an aqueous emulsion with a second ethylenically unsaturated monomer. The Amick process, although having many advantages, produces a graft copolymer having linkages, located between the side chains and backbone, that are susceptible to hydrolysis under certain conditions. These linkages result from using a mercapto-olefin compound having an ester functionality in the preparation of the macromonomer.
U.S. Pat. No. 5,264,530 to Darmon et al. (“Darmon”) discloses an emulsion or suspension free radical polymerization process where one or more monomer species is polymerized in the presence of a macromonomer that is used as a chain transfer agent. As the macromonomer is being used as a chain transfer agent, the macromonomer is predominately incorporated into the polymer chain at the ends.
U.S. Pat. No. 5,804,632 to Haddleton et al. (“Haddleton”) discloses an aqueous polymer emulsion process that includes preparing, in the presence of a cobalt chelate complex, a low molecular weight polymer having acid functional groups, and subsequently polymerizing at least one olefinically unsaturated monomer in the presence of the low molecular weight polymer to form a hydrophobic polymer. The low molecular weight polymer in Haddleton is taught to contain a sufficient concentration of acid to render the low molecular weight polymer, as is, or upon neutralization of the acid groups, partially or more preferably fully dissolvable in an aqueous medium. Although Haddleton discloses that some degree of grafting may occur, Haddleton focuses on processes where it is believed that the hydrophobic polymer particles are encapsulated by the low molecular weight polymer in the form of an “inverted core-shell” latex, or where the low molecular weight polymer serves simply as a seed for the polymerization to form the hydrophobic polymer. Thus, Haddleton does not disclose a process to produce graft copolymers of a desired structure such as comb copolymers. It is also believed that using the Haddleton process undesirably results in a substantial amount of low molecular weight polymer remaining unreacted in the water phase.
Publication WO 99/03905 to Huybrechts et al. (“Huybrechts”) discloses an anionically stabilized graft copolymer composition that is prepared by emulsion polymerizing acid containing macromonomer and amino functional monomer. The copolymer composition prepared contains from 0.5 to 30 weight percent amino functional monomer in the polymer backbone, and at least 5 weight percent acid functional monomer in the macromonomer that is neutralized with an amine. However, it would be desirable to provide an aqueous emulsion polymerization process for preparing alternative graft copolymer compositions that do not require neutralization.
The present invention seeks to provide a robust emulsion polymerization process for preparing graft copolymers that are preferably resistant to hydrolysis. The present invention also seeks to provide an emulsion polymerization process that preferably provides control over such parameters as polymerization kinetics, polymer structure, conversion, incorporation of macromonomer and particle size.